Fallon wrote to Prime Minister Theresa May, telling her he was resigning from her front bench cabinet, Xinhua news agency reported.

Downing Street confirmed the resignation, saying that May had written to the defence chief, accepting his resignation.

May wrote to Fallon: “I appreciate the characteristically serious manner in which you have considered your position, and the particular example you wish to set to servicemen and women and others.”

She praised the way he had championed Britain’s armed forces during his three and a half years as Defence Secretary, and the way he had held the job during a time when Britain faced threats of terrorism.

Political commentators said his resignation will come as a major blow to May, losing one of her most supportive and dependable ministers.

The move came just hours after May told MPs in the House of Commons that she had called a meeting of the main party leaders next week to discuss how parliament should handle complaints about sexual harassment by politicians at Westminster.

Earlier this week, Fallon admitted that he had put his hand on the knee of a radio presenter at a dinner 15 years ago at a Conservative Party conference.

In his letter to the Prime Minister, Fallon said: “A number of allegations have surfaced about MPs in recent days, including some about my previous conduct.”

“Many of these have been false but I accept in the past I have fallen below the high standards that we require of the Armed Forces that I have the honour to represent. I have reflected on my position and I am therefore resigning as Defence Secretary.”

He added in a statement: “It has been a privilege to have served as Defence Secretary for the last three and half years, and I have nothing but admiration for the professionalism, bravery and service of those men and women who keep us safe.”

The broadcaster at the centre of the knee-touching incident, Julia Hartley-Brewer, had spoken about the incident, but she did not reveal his identity.

“I have had no issues since with the man in question and do not regard the incident as anything but mildly amusing, which is why I have declined to name him.”

The Daily Telegraph said the resignation has come amid a growing sexual harassment scandal at Westminster, with some 40 Conservative MPs being named in a so-called “dirty dossier” compiled by party staff of politicians against whom accusations have been made.